Porous gas diffusion elements have been used since the 1920's for bubbling air into sewage in the activated sludge process.
Such elements are formed of a body of solid particles which has been shaped, pressed and rendered coherent by bonding or sintering in a compacted form having pores. Such compacts have been manufactured in a variety of forms of plate or disc-like configuration and mounted in holders in or near the bottom of sewage treatment tanks. Air under pressure from a plenum beneath the element is forced upwardly through pores extending through the body of the element to its upper surface, from which the air is released in the form of bubbles whose fineness is controlled in part by the sizes of the pores at the upper surface. The air encounters some resistance in passing from beneath the element into the water, and it is widely known that this includes frictional losses resulting from passing the air through the fine pores of the element.
With the exception of occasional defective elements which are inevitably produced in most manufacturing processes, the manufacturers of diffusion elements have apparently assumed that the quantity of air released from the upper surfaces of such elements was distributed with reasonable uniformity across the entire surfaces of the elements. Although highly detailed design and performance specifications are regularly applied to most components of sewage aeration systems, stringent distribution uniformity specifications have not been developed for diffusion elements. Also, a widely accepted test for uniformity of air distribution in a sewage aeration air diffusion element has been to merely make a visual examination of the bubble pattern emitted by the element while it is operating submerged in water. Moreover, persons skilled in the art have accepted this type of test for many years. They have done so despite the fact that it is quite difficult to visually ascertain whether a submerged diffusion element is distributing air uniformly, due to the disturbance created by discharge of bubbles into the water. Moreover, if accurate methods have existed heretofore by which one could compare the air output of different portions of the area of a diffusion element, such techniques have not been generally known and applied in commerical practice by persons active in the manufacture of sewage aeration diffusion elements and associated aeration systems. From this it might appear that there is little or no need for detailed or stringent specifications for the air flow distribution uniformity of diffusion elements for aeration.
A bubble release pressure test developed by the present applicants has made it possible to compare the relative ease with which different portions of the gas discharge surface of a diffusion element will discharge bubbles. Through the use of this test it has been found that the gas distribution properties of diffusion elements are not nearly as uniform as previously supposed. Although randomly disposed disuniformities of gas distribution have been observed, use of the bubble release pressure test referred to above has shown a trend for some diffusion elements to discharge a disproportionate amount of their total flow through certain zones. A larger quantity of flow through a given zone results in an increased rate, which tends to produce larger bubbles. Due to their reduced surface area per unit volume, larger bubbles tend toward reduced gas transfer efficiency, e.g. OTE, oxygen transfer efficiency. Thus, in a sewage aeration process, passing a disproportionate share of the total flow through a given portion of the diffusion element, while another zone of the element is underused, produces excessively large bubbles and therefore reduced oxygen transfer efficiency.
For a number of reasons, it can be useful or desirable to form a diffusion element with a peripheral zone having lesser permeability, greater density or lesser height than portions of the element surrounded by said peripheral zone. For example, as disclosed in U.S. Pat. No. 4,046,845 to Richard K. Veeder, a diffusion element may be provided with a substantially impermeable peripheral zone to prevent formation of coarse bubbles at the edge of the element. On the other hand, one may form a diffusion element with a step at its outer edge for seating an "O" ring or other sealing member(s), in which case the peripheral zone may be permeable, semi-permeable or substantially impermeable. If such a step is present, it can include a generally or substantially horizontal surface overlying the peripheral zone and a generally upright or vertical surface adjacent said horizontal surface or peripheral zone.
Using the above described bubble release pressure test, it is possible to show that formation of a diffusion element of the character described, having a peripheral zone, with or without a step, can lead to emission of non-uniform flow patterns. Where such a step is present and its generally upright or vertical surface is in communication or contact with the liquid medium to be aerated, such surface can emit or cause the emission of coarse bubbles with the disadvantages described above. Where there is no such step, forces generated during pressing in portions of the element adjacent the peripheral zone can also lead to production of diffusion elements with non-uniform flow patterns. Thus, a need exists for improvements in diffusion elements to overcome these difficulties. The purpose of the present invention is to meet this need.